Activity Monitor

I finally made the move to OS X about a week ago when I recieved my new dual 2.5. Quite a jump from my old machine, and I am very happy with the purchase with no problems to report. The only strange thing is the activity monitor. I have 4.5 GB of ram, but every once in a while it says I have 512 MB. TechTool and the System Profiler always report the correct amount, and in the slots I installed them in. I was worried there might be a compatability problem with the ram I bought from OWC. Then last night the Activity Monitor said I had only 1 cpu. Again, Tech Tool and system profiler show everything is as it should be. Sorry for the long post, just wondering if this is a know bug (10.3.5) or something wrong with my new computer?! Thanx.